People generally prefer daylight over artificial light as their primary source of illumination. Everybody recognizes the importance of daylight in our daily lives. Daylight is known to be important for people's health and well-being.
In general, people spend over 90% of their time indoors, and often away from natural daylight. There is therefore a need for artificial daylight sources that create convincing daylight impressions with artificial light, in environments that lack natural daylight including homes, schools, shops, offices, hospital rooms, and bathrooms.
Artificial daylight sources on the market focus mainly on high intensity, tunable color temperature, and slow dynamics (day/night rhythms). It is also known to create a sky view in a ceiling using a display or foil.
There has been significant development of lighting systems which try to emulate daylight even more faithfully.
Current technology used to create daylight effects is often based on fluorescent solutions with a strong diffuser on top. It is possible to create tunable intensity and tunable color temperature solutions using this approach. However, many of these solutions do not provide a realistic daylight experience because there is hardly any direct light to provide sharp shadows. Indeed, one particular feature of natural daylight which has not been well emulated is the relation between diffuse and direct light. Direct light provides sharp shadows whereas diffuse light is less intense. An impression of natural daylight is much stronger when direct light and diffuse light components are combined. This issue has been recognized, and artificial skylight systems have been proposed that simulate a number of daylight features, including for example the sky appearance with blue diffuse light and white direct light.
For example, it has been proposed to create a blue (i.e. clear sky) appearance when a user looks at the skylight at an angle (i.e. typically 40-90 degrees offset from the normal downward direction, which is the typical viewing angle range for a skylight), but still emits mainly white light in an angular area 0-40 degrees from the normal of the skylight surface, i.e. downward. This white light provides functional lighting. This approach is based on the combination of two main elements:    (i) an area light source to create an area of uniform white light;    (ii) a blue tubular grid that lets through the white light of the area light source in the direction perpendicular to the exit window unaltered, while filtering the light by an increasing degree for directions deviating from the perpendicular direction. The filtering renders the light blue.
In respect of the area light source, one approach is to use a direct lit mixing box to create a uniform white light source in combination with a microlens optic (MLO) plate to shape the light. Another approach is to use an edge lit lightguide with outcoupling structures to create a uniform backlight.
One problem with this approach generally is that the optical efficiency may be low since the uniform area light sources used have a rather wide beam, resulting in a large fraction of the beam being absorbed by the blue grid. This results in the over-installation of LEDs to compensate for this and reach the desired light levels. The problem arises because it is challenging to create both a uniform area light source and which is also collimated.
Another problem is that the white light is not extremely well collimated, for example with an approximate beam width of about 2×30 degrees, and therefore does not give the impression of direct sunlight in the room.
An additional requirement of all solutions is that the total system should have a limited depth, so it can be installed in existing buildings without the need for considerable structural modifications to the buildings.
There is therefore a need for a lighting system design which is able to provide a blue appearance when looking at a grazing angle into the luminaire, whereas a relatively highly collimated warm white light output is provided for task light, that creates a highly uniform spot corresponding to the shape of the luminaire. There is a need for a system which can achieve these aims with limited depth, for example less than 10 cm.